The present invention generally relates to an image forming system, and particularly to an image forming system related to a printer system such as a laser beam printer, a light-emitting device (LED) printer, a liquid crystal shutter printer or an ink jet printer, or related to a high performance copying machine or a high performance facsimile machine.
Currently, an image forming system is formed as "a system". Generally, an image forming system such as a laser beam printer is made up of a main body of the system (system main body), and various optional units which are detachably attached to the system main body. A system main body receives character code information or image information supplied from a host system coupled to the image forming system, and forms an image on a recording medium. A word processor, an office computer or a personal computer may be used as a host system. Another type of the system main body is known which directly receives image information from a host system. In this case, the image forming function is provided in the host system. Optional units are coupled to the system main body through respective interfaces. Those examples of optional units are a recording medium feed unit such as a large-capacity paper feed unit or a multistage (multi-tray) paper feed unit, a recording medium transporting unit such as a paper transporting unit for use in duplex print, and a recording medium eject unit such as a large-capacity paper eject unit or a multi-tray paper eject unit.
Conventionally, every optional unit is independently designed. An interface between an optional unit and the system main body is different for different optional units. Further each optional unit is connected to a system main body one by one in parallel form, as shown in FIG. 1.
In conventional image forming systems as mentioned above, all specifications related to the system main body and optional units connected thereto must be determined at an initial stage of system design. Additionally, there is no interchangeability between different optional units used in different image forming systems. Further, since every optional unit is connected to the system main body one by one, a limited number of optional units can be connected thereto. Once the system design is completed, the number of usable optional systems is limited. This is poor in flexibility and system expansion.
There has been proposed an image forming system which has a system main body capable of discriminating operable optional units connected thereto by using a registered identification code specifically assigned to each optional unit. However, the system main body cannot discriminate an optional unit having an identification code other than the registered identification codes. In other words, a command system employed in conventional image forming systems is lacking in flexibility and is not suitable for system expansion.
For example, a time it takes the leading edge of paper to reach a registration sensor from the start of paper feed from an optional paper feed unit is predetermined at the stage of system design. Such a time is predetermined for each of the available optional paper feed units. For this reason, an optional unit specifically designed to a system is not allowed to be used for a different system.